Conventionally, there is a known surface-coated cutting tool including a base material and a coating film formed on the base material, in which a TiB2 layer is included as the coating film.
For example, Japanese Patent Laying-Open No. 51-148713 (PTD 1) discloses a wear-resistant molding member comprising a cemented carbide base body and a surface layer that is formed of two laminated partial layers including an outer partial layer made of aluminum oxide and/or zirconium oxide, and an inner partial layer made of one or more types of borides, particularly, diboride that is an element of titanium, zirconium, hafnium or the like (that is, a TiB2 layer).
The inner partial layer of the above-described surface layer forms a TiB2 layer of 3 μm that is obtained under high-temperature and high-vacuum conditions of 1000° C. and 50 torr by introducing hydrogen at 1900 l/hour, TiCl4 at 20 ml/hour and BCl3 at 4 g/hour each as reactive raw material gas, and forming a film for one hour. Also, the outer partial layer forms an aluminum oxide layer of 5 μm.
However, under the high-temperature and high-vacuum conditions during the film formation as mentioned above, diffusion of boron in a junction layer and a TiB2 layer into the cemented carbide base body causes production of a strong η layer and/or a boron-containing brittle layer, thereby significantly reducing the lifetime of this wear-resistant molding member.
For the purpose of solving the above-described problems, a coated product is proposed that is improved in wear resistance by suppressing diffusion of boron and finely granulating TiB2 in the TiB2 layer (Japanese Patent National Publication No. 2011-505261 (PTD 2)). This coated product is formed by coating the surface of the cemented carbide base material with a layer of 0.1 μm to 3 μm made of a group of titanium nitride, titanium carbonitride and titanium boron carbonitride, and then, forming a TiB2 layer of 1 μm to 5 μm. The conditions of forming the TiB2 layer of various layers mentioned above are as follows. Specifically, a TiB2 layer having a thickness of 2.5 μm is formed by the thermal CVD method for 1 hour at a standard pressure and at a temperature of 800° C., using a raw material gas composition including 10% by volume of hydrogen, 0.4% by volume of TiCl4. 0.7% by volume of BCl3, and 88.9% by volume of argon gas. In this coated product, a boron-containing brittle layer caused by diffusion of boron into the cemented carbide base material is not formed, and the particle size of TiB2 in the TiB2 layer is also controlled to be 50 nm or lower, so that the lifetime of the tool is improved to some extent.